BANK EROSION

Bank erosion refers to the detachment of bank soil resulting from hydraulic and geotechnical forces, and is commonly categorized into flow-induced bank erosion and bank collapse. Bank collapse occurs when the driving forces that tend to move soil downslope (e.g., the weight of soil and plants, positive pore water pressure, and seepage forces) exceed the resisting forces of the bank (e.g., soil and root cohesion, matric suction, hydrostatic pressure, and vegetation roots). On the basis of failure patterns, three principal mechanisms of bank collapse are defined as shear, toppling, and tensile failure. Bank erosion is of fundamental importance to the morphodynamics of fluvial, estuarine and coastal environments, affecting a wide range of physical, ecological and socio-economic processes. Bank erosion drives channel width changes, serves as a major source of sediment load creating riparian habitats (e.g., floodplain and alternate bar), and induces farmland and wetland loss. 

Here, we set up a laboratory experiment to reproduce flow-induced bank erosion and bank collapse, and to study the role of bank height (Hb) and near-bank water depth (Hw) on bank stability. Results show that the patterns of bank failure can be related to Hb/Hw . For large Hb/Hw(>=2), we observe a cantilever-shape bank profile. For small Hb/Hw (<2), we first observe cracks on the bank top, followed by shear failures along a vertical or inclined surface separating the cantilever block up from the bank top. When accounting for our results in the context of previous experimental studies, we find a transition point characterized by a maximum normalized bank retreat rate. For toppling failures, we also find a positive correlation between the ratio Hb/Hw and the geometrical contribution to bank retreat from bank collapse (Cbc).

To simulate the geomorphodynamic evolution of tidal channels, we develop a process‐based model considering hydrodynamics, flow‐induced bank erosion, gravity‐induced bank collapse, and sediment dynamics. A stress-deformation analysis and the Mohr-Coulomb criterion are included in a model simulating bank collapse. Results show that collapsed bank soil plays a primary role in the dynamics of bank retreat. For bank collapse with large bank height, shear failure at the bank toe (stage I), tensile failure on the bank top (stage II) and sectional cracking from bank top to the toe (stage III) are present sequentially before bank collapse occurs, agreeing well with the experimental observations. The bank profile is linear or slightly convex and the planimetric shape of tidal channels (gradually decreasing in width landward) is similar when approaching equilibrium, regardless of the consideration of bank erosion and collapse. 

Overall, this project quantifies the role of Hb/Hwon bank collapse, bank retreat rate, and the overall Cbc, and highlights the importance of bank collapse and the resultant collapsed bank soil in investigating tidal channel morphodynamics using a combined perspective of geotechnics and soil mechanics. 

Contact: Kun Zhao at kunzhao1357@gmail.com   

Gong, Z., K. Zhao, C. Zhang, W. Dai, G. Coco, and Z. Zhou (2018), The role of bank collapse on tidal creek ontogeny: A novel process-based model for bank retreat, Geomorphology, 311, 13-26.

Zhao, K., Z. Gong, F. Xu, Z. Zhou, C. K. Zhang, G. Perillo, and G. Coco (2019), The role of collapsed bank soil on tidal channel evolution: A process‐based model involving bank collapse and sediment dynamics, Water Resources Research, 55(11), 9051-9071.

Zhao, K., Z. Gong, K. Zhang, K. Wang, C. Jin, Z. Zhou, F. Xu, and G. Coco (2020), Laboratory experiments of bank collapse: the role of bank height and near-bank water depth, Journal of Geophysical Research: Earth Surface, Doi: 10.1029/2019JF005281.